Our invention relates to a protective coupler device for insertion between a telephone station and the central office. Such protective couplers are used to insure that overvoltages which could occur as a result of a malfunction in the telephone station set or in the telephone connection system are not passed along the telephone lines to the central office.
There are two basic types of protective couplers. One type acts in much the same manner as does a fuse in that upon detection of a hazardous voltage the line is opened and remains open until the trouble condition is cleared. Such a device is shown in Gordon-Mazurek-Wright, Ser. No. 764,594, filed Feb. 1, 1977. The second type of protective coupler acts to prevent the overvoltage from being transmitted to the central office line by means of internal circuitry. The line remains connected but the hazardous voltage does not pass through a barrier. Such a circuit is shown in co-pending application of Angner-Egan-Gordon-Huryn 16-4-10-3, Ser. No. 840,593, filed Oct. 11, 1977.
A problem when such devices are used with key telephone line circuits is that at times some voltages which are over the prescribed limit must be passed over the line. An example of such a situation is the ringing voltage extended from the central office to the station. In some prior art, such "legitimate" ringing voltage is filtered from the detection circuit by a filter which blocks the low frequency usually associated with ringing voltages. Such circuits suffer from the problem that they can only work with the type of overvoltage generated between two lines, commonly called longitudinal overvoltages, and not with overvoltages which appear on only one line, called metallic overvoltage.
Accordingly, it is an object of my invention to provide a circuit for use with a key telephone line circuit to protect against both longitudinal and metallic overvoltages while still allowing certain overvoltages to appear on the line.
It is a further object to provide such a circuit that will continue to allow the circuit to function in the event of a power failure.